BIOCHEMICAL MODELS OF HETEROSIS IN NEUROSPORA 



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the hy})hal tips. To prove that two kinds of nuclei were present in the same 

 cells of such heterocaryons, Beadle and Coonradt (1944) cut ofT single hyphal 

 tips, transferred them to fresh medium, and then identified two kinds of 

 nuclei in the resulting growth by genetic test. 



Where there is freely branching filamentous growth, as in Neurospora, it is 

 possible for the two types of nuclei in a heterocaryon to become sorted out 



Fig. 12.2 — Somatic segregation of dissimilar nuclei in the formation of conidia (a diagram). 



purely as a matter of chance, as illustrated in a schematic way in Figure 12.2. 

 This diagram actually represents an erect fruiting branch, or conidiophore, 

 on which the asexual spores are born. The conidia of Neurospora have 

 variable numbers of nuclei, but generally more than one. Dodge (1942) 

 proved that two kinds of nuclei were present in the same cell of a heterocar- 

 yon by growing cultures from single conidia, and then showing by genetic 

 test that some of these cultures had both types of nuclei. In some instances 

 he was able to distinguish the heterocaryotic and both homocaryotic types 

 in culture derived from single conidia by their mon:)hological characteristics. 

 The essential differences between Neurospora and higher organisms with 



